This invention is concerned with utilization of plywood waste material available in conventional plywood manufacturing operations. Veneer produced for plywood production purposes must be cut to dimensions greater than the desired plywood sheet dimensions to permit final trimming of each plywood sheet and to minimize voids or recesses at the edges of the several veneer layers. Furthermore, all veneers are initially trimmed to the same dimensions whether they are to be used with their grain running along the length of the plywood sheet (as "centers" or "faces") or with their grain perpendicular to the length of the sheet (as "cross bands"). In order to leave an excess of at least 1 inch along any side of a sheet of veneer in a plywood sheet 4 by 8 ft. in size it is typical to produce veneer sheets approximately 100 inches in length or more along the grain direction. The 4 inch excess in veneer length permits cutting of the veneers for use as cross band material. A typical veneer sheet width is approximately 50 inches or one-half the full veneer length. As a result, there is a trim strip of approximately 1 inch at each side of a plywood sheet and approximately 2 inches at each of its ends. Because the end trim material equals approximately 4% of the total veneer utilized at the plywood mill, substantial material salvage can be accomplished by utilization of end trim. Plywood edge trim conventionally cannot be used except as fuel.
Prior U.S. Pat. No. 2,572,772 is directed toward use of trimmings from plywood sheets, teaching the layup of the sheets into panels with the veneers parallel and face glued. This results in a panel having substantial areas of end grain surface, more analagous to laminated wood beams than to plywood, which typically has large areas of longitudinal and transverse grain veneer material.
Another patent of interest is Alenius U.S. Pat. No. 3,133,850, which is concerned with continuous plywood production, including use of lumber strips as core materials. It has no direct teaching concerning salvage of plywood trim strips.
According to this disclosure, plywood trim strips are edge glued to produce a core panel having coplanar surfaces at each side similar to the conventional face surfaces of a plywood panel, but composed of discontinuous veneer strips. This core panel is then used as the center of a thick plywood sheet by addition of cross bands and face veneers at each side of the core panel.
Besides the benefits of salvaging substantial amounts of the total veneer used in plywood production, the process has the further advantage of permitting the pressing and curing of thick plywood panels in a greatly reduced time, since the pressing requires curing of only the glue lines outward from the prepared core panel. In contrast, preparing identically thick plywood panels by conventional plywood layup methods requires curing of all the glue lines, which greatly slows the production of plywood. The time required for complete heating of plywood in a press increases with the number of plies and total sheet thickness. Likewise, producing thick plywood panels by face gluing two equally thick panels or by gluing them to a center cross band also requires heating of the center glue lines through the bulk of the entire assembly.
The plywood product that results from this process is much thicker than average plywood and has substantially the strength properties of conventional plywood of the same thickness. The strength of plywood in normal applications reflects mostly the strength of the outer one or two plies, which in this product are of conventional structure and quality.